Stars: (f) characteristics of a neutron star and a black hole

The idea

A neutron star is the collapsed core left after a supernova. It is extremely dense, with a mass similar to the Sun packed into a radius of about 10 km, and it is supported against further collapse by neutron degeneracy pressure. Many neutron stars spin rapidly and emit beams of radiation, detected as pulses when the beam sweeps past Earth. A black hole is a region where gravity is so strong that the escape velocity exceeds the speed of light, so nothing, not even light, can escape from inside the event horizon. Its observable effects come from its gravitational pull on nearby matter and light.

What earns the marks

• A neutron star is extremely dense, with a mass comparable to the Sun in a radius of roughly 10 km.
• Neutron stars are supported by neutron degeneracy pressure and may be observed as pulsars.
• A black hole has an escape velocity greater than the speed of light.
• The event horizon marks the boundary inside which nothing can escape, including light.

Common mistakes and how to put them right

• Confusing a neutron star with a white dwarf.

  Put it right: A white dwarf is supported by electron degeneracy pressure and is much less dense.

• Thinking a black hole can be seen directly.

  Put it right: It is detected through its gravitational effects on nearby matter and light.

• Believing a black hole has a solid surface.

  Put it right: The event horizon is a boundary in space, not a material surface.

Exam tips

• Compare the two objects using density, size and escape velocity.
• Use the phrase event horizon when describing the boundary of a black hole.
• Remember that a pulsar is a rotating neutron star emitting beams of radiation, not a separate type of remnant.